Electroacoustic transducers, and in particular electrodynamic transducers, are widely used in telecommunications equipment such as wired and mobile telephones, where small size is a requirement. Traditional electrodynamic microphones and speaker transducers used in e.g. mobile telephones are rotational symmetric and have a circular disc or ring shaped permanent magnet, which is magnetised in the axial direction of the magnet. A magnetic circuit of magnetically soft iron or other suitable material define a ring-shaped gap with a radially oriented magnetic field created by the magnet. A diaphragm carries a ring-shaped coil of electrically conducting wire situated in the gap.
If the inner and outer members defining the gap are not perfectly coaxial, the gap will not have a uniform width resulting in a distorted distribution of the magnetic field along the gap. A coil carrying electric currents at audio frequencies in such a distorted magnetic field will tend not to move in a linear movement but to tilt, which causes linear and non-linear distortion.
In such transducers the magnetic field in the ring-shaped gap is radially oriented, whereby the magnetic field is inherently stronger at its inner limit than at its outer limit. A not perfectly centred coil will cause the same distortion as mentioned above.